1 . Field of the Invention
This invention relates to cellular telephone systems and, more particularly, to processes for determining those base stations with which a mobile unit should communicate in code division multiple access (CDMA) cellular telephone systems.
2 . History of the Prior Art
Presently available commercial mobile communication systems typically include a plurality of fixed base stations (cells) each of which transmits signals to and receives signals from mobile units within its communication area. Each base station in a CDMA system communicates with mobile units by sending digital transmissions over the same frequency spectrum. In most cellular systems, especially those with cells in urban areas carrying heavy traffic, each base station may be further divided into two or three sectors each of which may include its own transmission equipment the antennas of which provide 180 or 120 degree coverage, respectively. When the term base station is used herein, both sectors and cells are intended unless the context indicates otherwise.
A CDMA system transmits messages digitally. All transmissions in a CDMA system are on the same frequency spectrum so the digital signals constituting each message must somehow be recognizable from all of the possible transmissions available. To accomplish this, the digital messages are encoded by a series of overlaid digital codes. One of these codes, called a pseudorandom noise (PN) code, is applied to all transmissions throughout a CDMA system. The PN code is used to encode the individual bits of the original message at the transmitter and to decode the encoded message at the receiver. In order to recognize messages from a particular base station, each base station uses a distinct time offset (called a PN offset) from some repeating initial time to begin encoding the transmission using the PN code. Thus, one base station may begin an encoded transmission at the initial time, a second base station at an offset of one unit from the initial time, a third at an offset of two units from the initial time, and so on up to a total of 512 offsets.
Each transmission between a mobile unit and a base station is also placed on what is effectively a separate channel by further encoding the transmission with one of a plurality of Walsh codes. A message encoded by a Walsh code, as with a PN code, can only be decoded by the same Walsh code at the receiver. Thus, an encoded transmission on a particular channel is decoded by applying a mask including both the Walsh and PN codes to the received pattern of information bits commencing at the PN offset designated for the particular base station.
A base station normally has sixty-four Walsh codes available for defining channels on which it can establish transmissions with mobile units. Certain of these channels are preassigned to function as control channels. For example, in order to advise mobile units of the particular PN offset used, each base station continuously broadcasts the PN code using its assigned PN offset on one of these channels (a pilot channel) defined by Walsh codes. Mobile units monitor this preassigned pilot channel. When a mobile unit finds an offset at which a pilot is decodable, it refers to another control channel (a synchronization channel) to determine the initial time and thereby identify the PN offset of the base station. Each system also maintains a paging channel upon which indications are posted that new messages are arriving. A total of nine channels are provided for these and other control functions.
In order to allow mobile units to transmit and receive telephone communications as the units travel over a wide geographic area, each base station is normally physically positioned so that its area of coverage is adjacent to and overlaps the areas of coverage of a number of other base stations. When a mobile unit moves from an area covered by one base station to an area covered by another base station, communication with the mobile unit is transferred (handed oM from one base station to another base station in an area where the coverage from different base stations overlaps.
In most other types of cellular communication systems, a mobile unit communicates with only one base station at a time. However, since all transmissions in a CDMA system take place on the same frequency spectrum, a mobile unit actually has available all of the information which is within its range. However, it only decodes information on PN offsets and Walsh code channels which are directed to it. A CDMA mobile unit uses a receiver which is able to apply a number of decoding masks simultaneously at different offsets of the entire spectrum of information which it receives. At present, a mobile receiver may decode as many as six PN offsets at once. However, usually only three PN offsets are used to decode messages while the others decode control information. Because a mobile unit in a CDMA system may be receiving the same information from a number of different base stations at the same instant, it may decode information from a single message sent to it from a number of different base stations simultaneously using different PN offsets and Walsh codes and combine that information to produce a single output message. Thus, while a signal transmitted from one base station may be fading, the same message may be being received with adequate strength from another base station. This allows a CDMA system to offer the possibility of significantly better transmission. The situation in which a mobile is communicating with a number of base stations at once is called xe2x80x9csoft handoff.xe2x80x9d
In order to utilize the advantages offered by CDMA technology, a mobile unit must be able to select the correct base stations with which to communicate. However, it is much too slow for a mobile unit to interrogate each of the 512 PN offsets in order determine those base stations with which it should be communicating. Consequently, a mobile unit uses a xe2x80x9cneighbors listxe2x80x9d which specifies those PN offsets at which transmissions are most likely to occur in order to reduce the time required for its search for transmissions. A neighbors list is furnished by a base station with which a mobile unit is in contact.
The method by which a mobile unit decides whether to make contact with a particular base station is quite complicated. The mobile unit continuously monitors the strength of signals broadcast on the pilot channel by the base stations to try to utilize the highest quality signals possible. The pilot signals for base stations with which the mobile unit is presently in contact are monitored most frequently, followed by pilot signals for base stations on the neighbors list, followed by pilot signals for all base stations as a group. When the strength of the pilot signal from a particular base station rises above a certain threshold level (Tadd) with respect to the level of all signals being received by the mobile (the interference received in spite of encoding), the mobile issues a request to the system and, upon direction, one of the receiving elements is allotted to that base station. This is called putting the base station on the active list. When the strength of the pilot signal from a base station falls below another threshold level (Tdrop) with respect to the level of all signals being received by the mobile for a set period, the mobile issues a request to the system and, upon direction, the base station is removed from the active list so that the receiver may be allotted to another stronger base station. Moreover, if a pilot signal is stronger than the weakest pilot signal of a base station in the active set, the mobile issues a request to the system and, upon direction, the stronger base station will replace the weaker base station.
In order for a system operator to allocate resources to a CDMA system intelligently, the operator typically models the system to determine where changes should be made. One of the criteria which is important in determining the allocation of resources is a determination of the base stations with which a mobile unit is in contact at any particular location. Most aspects by which the efficiency at which a system is operating depend on this determination since the active list base stations determine the level of signal compared to interference at any location.
Although the actual method by which a determination of the base stations with which a mobile unit is in contact is quite complicated, all prior art models simply assume that any base station providing a pilot signal greater before decoding than some cutoff level with respect to the total of all signals received by the mobile unit will be in active communication with the mobile unit. This leads to incorrect modeling of the system and the incorrect allocation of assets.
Consequently, it is desirable to provide a new process by which the handoff properties of a CDMA cellular system may be modeled so that steps may be taken to improve the system.
The present invention is realized by a computer implemented process which determines those base stations which most probably communicate with a mobile unit at a particular location, determines a probability for each base station to communicate with a mobile unit based on its actual received signal level compared to the interference level of signals received at the location, computes for each set of base stations a probability of that set of base stations communicating with a mobile unit at the location, sums the probabilities of a base station and any other base station serving together throughout the system for each other base station, and selects from the highest sums of probabilities a list of neighbor base stations for each base station at each location.
These and other features of the invention will be better understood by reference to the detailed description which follows taken together with the drawings in which like elements are referred to by like designations throughout the several views.